The present invention relates to a process for the separation of americium from the curium contained in an aqueous nitric solution resulting more particularly from the reprocessing of irradiated fuels.
It is known that the separation of americium from curium is a very difficult operation, due to the very similar chemical properties of these two elements. Numerous processes have been tested with a view to carrying this out.
Among the processes for separating americium from curium known up to now, reference is made to the reversed phase chromatography method according to which a column containing an inert support, for example teflon is used and onto this is passed a relatively volatile solvent which acts as the extracting agent. The americium is separated from the curium contained in the solution passed into the column by ion exchange. This reversed phase chromatography process makes it possible to obtain a good separation of the americium from the curium, but is difficult to use for large-scale production purposes due to difficulties encountered during the preparation and hydraulic operation of the columns.
Another hitherto tested and tried process for the separation of americium from curium involves separation on ion exchange resin (mineral or organic exchangers). According to this method, separation is carried out by a selective elution of the americium and the curium at valency III, the quality of separation being a function of the nature of the eluent. Therefore, several different eluents have been envisaged, for example a hydrolalcoholic solution of nitric acid, an ammonium .alpha.-hydroxyisobutyrate solution, a hydroalcoholic solution of nitric acid containing ammonium nitrate and DTPA (diethylene-triaminopentaacetic acid), a hydroxy-ethyl-diaminotriacetic acid solution, etc. According to this process, the separation factor of the americium from the curium remains low and is of the order of 2 to 3. The same separation method on ion exchange resin, but under pressure makes it possible to more efficiently and rapidly separate americium from curium. However, it is difficult to perform, due to the high pressure used (pressures close to 300 atmospheres).
Another prior art process for separating americium from curium is the extraction by solvent of elements at valency III by means of amines in a salting-out medium (which consists of adding beforehand salting-out salts such as aluminium nitrate or lithium nitrate to the aqueous solution to be treated). After extraction of the elements at valency III, a selective re-extraction can be obtained with a polyaminoacetic complexing agent such as DTPA. This process has the disadvantage that the separation factors for the separation of the americium from the curium are only about 2 to 3.
Another process for separating americium from curium involves the selective precipitation of one of the constituents. The curium at valency III can be precipitated in fluoride or oxalate form in the presence of americium at valency VI or the americium can be separated by precipitation of the double americium and potassium carbonate. However, this method of precipitating one of the constituents only makes it possible to obtain very limited separation factors, so that the same operations have to be repeated several times to obtain the desired purity of one of the elements.
Consideration has also been given to the separation of americium from curium by liquid extraction by oxidizing the americium at a higher valency and then extracting it in an organophosphorus solvent constituted by di(2-ethylhexyl)-phosphoric acid. However, the results obtained are not satisfactory because it is difficult to maintain americium in the oxidized state. However, the results can be improved by using bis(2,6-dimethyl-4-heptyl)-phosphoric acid as the solvent, as described in U.S. Pat. No. 3,743,696. However, it should be noted that according to the latter patent, it is necessary to heat the solution in order to completely oxidize the americium.